According to the prior art, an apparatus for injection molding of plastic material comprises an injection machine of a screw plunger type and a mold arrangement defining a mold cavity where a molded article is formed. The mold arrangement may be a single mold for a molded product or consist of a primary mold for a molded product and a cold runner mold for a runner. In place of the cold runner mold, the apparatus may be provided with a hot runner mold incorporated with a manifold between the single mold and the injection machine. A nozzle passage is formed between the mold arrangement and a body of the injection machine, by a nozzle, in the most simple one cavity case or a hot runner mold arrangement and the nozzle.
With such an apparatus, a conventional injection molding process comprises steps of:
having a plastic material, in every shot cycle, plasticized and metered while being heated within the machine body;
having the hot plasticized material injected under pressure for the mold cavity through the nozzle passage;
having the hot injected material held at least partially within the entire mold cavity under pressure while the mold arrangement is being cooled to thereby provide and freeze a molded article therein; and
having the frozen molded article removed from the mold cavity after the mold arrangement is opened.
For example, particular steps of injection molding are indicated in Table I or II attached hereto including the above fundamental steps.
Among the important factors influencing the quality of a molded product, it has been recognized that the material pressure-holding step is one of the most critical steps. If this step is not carried out in a suitable manner, the molded products have undesired shrinkages due to short shot and/or flashes due to over-packing.
The time of the material pressure-holding step depends on the time needed for cooling a molded article in the mold cavity.
With a fixed mold cooling capability and a fixed volume of the mold cavity, a thinner molded article takes a shorter time than a thicker molded article to freeze it enough for removal from the mold cavity. In this connection, the thicker molded article requires more time in the material pressure-holding step, although the injecting step and the plasticizing and metering step each take the same time as for the thinner molded article.
According to the prior art, the material pressure-holding step is carried out using the injection machine with the plunger exerting an external holding pressure, subsequent to an injection pressure, on the injected material in a combination of the mold cavity and the nozzle passage against the mold cavity. The conventional technology involves a external holding pressure exerted by the injection machine being controlled to form a multi-stepped pressure, rather than a non-stepped pressure, which is stepped at predetermined strokes of the plunger.
Since a function and effect of the material pressure-holding has not yet been precisely known, although there are some theories which have been developed, there have been various attempts to improve the material pressure-holding step for a plasticized material which is viscid and elastic in the multi-stepped external pressure approach. In the conventional approach, as matter of course, there is a serious difficulty in controlling such a multi-stepped pressure stepped in an accurate manner at predetermined precise stroke positions, since a time between the neighboring steps of the pressure is very short on the order of 0.01 second and with a very short distance between the strokes at the neighboring pressure steps being on the order of 0.1 mm, while an inner diameter of a barrel of the injection machine body is very large relative to such a small stroke difference. Under these circumstances the multi-stepped pressure control cannot rely on a manual operation. Therefore, the recent injection machines for producing precise products, particularly small size articles, are all equipped with an expensive computer incorporated with expensive electronic detectors for the controlled parameters. In connection with the above, such a computer is also used for controlling the plasticizing and metering step and the injection step. In the injection step, there is also adopted a multi-stepped injection method involving a multi-stepped injection speed in most of the cases.
Further, it should be noted that only after the material pressure-holding step in a shot cycle is completed, can the plasticizing and metering step be carried out for a next shot cycle. This is because the injection plunger of the machine per se is essentially engaged in said material pressure-holding step. This means that the plasticizing and metering step is allowed to take a time from the completion of the material pressure-holding step to the time when the mold arrangement is opened for removing a molded article. This time is relatively short in a shot cycle period of time, for example 4.6 sec. (34%) in the shot cycle of 14.50 sec. as indicated in Table I, while the time of the material pressure-holding step takes 5.09 sec. (35%).
The shot cycle time (14.50 sec.) is a sum of the plasticizing and metering time (4.6 sec.), the material pressure-holding time (5.09 sec.) and the other steps (5.41 sec.).
There is, of course, a strong demand for a higher productivity of a precision article due to a shorter shot cycle time in the plastic injection molding industry. This demand, therefore, forces, in one way, the period of time of the material pressure-holding step to be shortened, while ensuring that the quality of molded precision article is still good. This causes not only the mold cooling capability to be improved but also the computer control of the multi-stepped holding pressure and injection pressure with the associated piston strokes to be improved with higher accuracy to harmonize with the improved mold cooling, with the result that the computer control per se is obliged to be a more sophisticated or complicated one with a higher cost incurred in computer equipment. At the present time, the following statement is by no means an exaggeration. The cost of the computer occupies a large part of the cost incurred in production of the injection machine with the result that recent machine production has become very expensive compared with the cost of the original simple injection machine used in the past, which was equipped with no computer and ran by a simple operation involving " a non-stepped pressure and a non-stepped injection speed." This naturally leads to a higher cost incurred in producing molded articles.
In the other ways, in order to reduce the production cost, various efforts to shorten the shot cycle time, of course, have been made in the industry. Such a shortened cycle time forces the time of the plasticizing and metering step to be shortened under the conventional circumstances. However, a serious problem is encountered in that shortening of the plasticizing and metering step requires an increase in the plasticizing rate or performance with an increased power supply, leading to an increased machine cost and operation cost.
Further, it should be noted that such an increased plasticizing capability of the injection machine causes the plastic material per se to be damaged by the screw plunger due to breakage of chains of a high resin polymer, while the polymeric material is being plasticized. This leads to deterioration of a molded article.
Still further, the increased plasticizing capability requires the material to be heated to a higher temperature. This leads to prolongation of the time required to cool the molded article in the mold, and thus the enhanced heating works against the attempt to shorten the shot cycle time. Due to this lower quality of the plasticized material and an enhanced heating, therefore, in combination, there is a certain limitation of enlarging the plasticizing capability, even if the increased cost incurred in production of such a higher performance injection machine is neglected.
Under the circumstances, the inventor recognizes that the improvement or development of the process and apparatus for injection molding of a plastic material in the conventional art is reaching or has reached the limit.